The present invention relates generally to plasma cutting systems and, more particularly, to a system for plasma cutting wherein a high-frequency starting circuit is integrated within a plasma-cutting torch. As such, the amount of high frequency, high voltage power necessary for starting the plasma cutter is reduced and the amount of wires and circuitry exposed to the power source is reduced. Furthermore, the plasma-cutting torch may be retrofitted to power sources regardless of a startup configuration for which the power source was designed.
Plasma cutting is a process in which an electric arc is used to cut a workpiece. Plasma cutters typically include a power source, a gas supply, and a torch. The torch or plasma torch is used to create and maintain the arc and plasma that perform the cutting. The plasma cutting power source receives an input voltage from a transmission power line or generator and provides an output voltage to a pair of output terminals, one of which is connected to an electrode and the other of which is connected to the workpiece.
The air supply is used with most plasma cutters to help start the arc, provide the plasma gas to the torch, and cool the torch. A movable or fixed electrode serves as a cathode and a fixed nozzle serves an anode. The air supply moves the electrode and as the electrode moves away from the nozzle, it opens the nozzle, and a plasma jet is created. The plasma jet causes the arc to transfer to the work piece, and thus initiates the cutting process. In other plasma cutting systems, a high frequency starter is used to initiate the cutting process.
Most plasma cutting systems implement one of two methods of initiating the plasma cutting arc: high frequency (HF) starting and the above-described contact starting. Contact start torches use a moving electrode or nozzle to create an initial spark to ionize the cutting gas and generate a pilot arc. A contact start torch begins with the electrode and nozzle in contact causing a short circuit, until the gas reaches the short, which blows the electrodes and/or nozzle apart to create a spark across the newly formed gap. The spark ionizes the gas thereby enabling current to flow across the newly formed gap between the nozzle and the electrode and create a pilot arc. While contact starting is a common method of initiating a plasma cutting process, it does present drawbacks. Specifically, the moving electrode and/or nozzle are particularly susceptible to wear. Furthermore, the designs are quite complex and are susceptible to misassembly, misalignment, or breakdown of the moving parts.
In contrast, HF starting is a method of generating a pilot arc without moving parts or the wear associated with shorting and breaking the nozzle and electrode. To perform HF starting, a plasma torch is connected to a power source having an HF starting circuit. The circuit typically includes a high-voltage transformer, capacitors for power conditioning, and a gap assembly to generate a high-voltage spark at the torch electrode. When sufficient power is transferred from the power source to the torch, a spark fires from the electrode and ionizes gas between the electrode and nozzle. This ionization enables current to flow across the air gap between the nozzle and the electrode. The result is a pilot arc to initiate cutting.
HF starting generally requires a voltage on the order of 3.5 kV to 5 kV to generate the ionizing spark. As such, the high-voltage transformer and coupling coil required to operate at such a voltage are relatively large, which can add to the size of the plasma-cutting system making portability cumbersome.
The high voltage must also be translated a considerable distance from the power source to the torch. It is not uncommon for distance between the power source and torch to reach distances of over fifty feet, resulting in significant losses. This problem is compounded as the distance from the power source to the torch varies as the cord connecting the two components is moved and stretched. Furthermore, extended lengths of the power cord or cable may be susceptible to relatively harsh working conditions that may negatively affect the HF power being transferred to the torch. For example, plasma cutters are commonly operated within large manufacturing environments or at an in-field site. Adding to these typically callous operating environments, the HF starting circuits typically generate a considerable amount of electrical noise. Such noise can be particularly undesirable in sensitive manufacturing processes where electrical interference can impact the operability of the manufacturing process.
Additionally, HF starting systems are mutually exclusive from contact starting systems. That is, because both the power source and the torch must be specifically tailored to the starting method, contact and HF starting power sources and torches may not be interchanged without modifications.
It would therefore be desirable to design a plasma cutting system that is capable of generating a pilot arc while reducing the aforementioned drawbacks of traditional contact and HF starting circuits. Specifically, it would be desirable to design a plasma cutting system with a HF starting circuit with reduced voltage requirements. Additionally, it would be desirable for the system to be able to be retrofitted to existing plasma cutting systems regardless of whether the system is designed for contact or HF starting.